Determination of the Absolute Rate Constant of Propagation for Ion Pairs in the Cationic Polymerization of p-Methylstyrene

Abstract

The cationic polymerization of p-methylstyrene (p-MeSt) was studied in dichloromethane, chloroform, and methylcyclohexane/methyl chloride 60/40 (v/v) at different temperatures with various initiators in conjunction with different Lewis acids, TiCl 4, BCl3, SnBr4, and SnCl4. Well-controlled living cationic polymerization was obtained in dichloromethane, in conjunction with SnCl4 as Lewis acid, and the living nature of the polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear number-average molecular weight (Mn) vs conversion plots in the temperature range of -15 to -70 °C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to M n ≈ 90 000 and agreed reasonably well with the calculated molecular weight, assuming that one polymer chain forms per molecule of initiator. The kinetics of p-MeSt polymerization suggests that the polymerization is first order with respect to SnCl4 concentration. Employing the model compound 1-chloro-1-(4-methylphenyl)ethane in conjunction with SnCl4 in CH2Cl2, UV-vis spectroscopy was used to follow the capping reaction with 2-phenylfuran to determine the equilibrium constant of ionization (K i) at -30 °C. From the Ki value and the apparent rate constant of propagation (kapp) the absolute rate constant of propagation for ion pairs, kp ( ) 6.8 × 10 8 L mol -1 s -1 at -30 °C, was calculated as a lower limit. The absolute rate constant of propagation for ion pairs, kp ( , was also determined at different temperatures from competition experiments, where polymerizations were carried out in the presence of 2-phenylfuran as capping agent. Gel permeation chromatography and NMR spectroscopy suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degrees of polymerization k p ( ) 1 × 10 9 L mol -1 s -1 was calculated using the known rate constant of capping. The kp ( value remained unaffected in the temperature range of -15 to -70 °C, indicating that propagation does not have an enthalpic barrier.

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization III: Polymerization of 1,3‐Pentadiene with CF3COOD/TiCl4 Initiating System: Chain‐Ends Structure and Kinetics

“…3 orders of magnitude higher than obtained earlier by Cheradame et al via kinetic simulations using Runge–Kutta method . On the other hand, the values of k p obtained here are much lower than those for the vinyl monomers of comparable reactivity (substituted styrenes) obtained using diffusion clock methods . Although the validity of the rate constants for propagation obtained by diffusion clock methods is questionable for less reactive monomers such as isobutylene and styrene and its derivatives, a good correlation between the k p values determined using the diffusion clock method ( k p = 11.4 × 10 4 L mol −1 s −1 at –30 °C), from an evaluation of ionic species concentration by spectroscopic methods ( k p = 2.8 × 10 4 L mol −1 s −1 at 0 °C) and those calculated based on PREDICI simulation ( k p = 4.9 × 10 4 L mol −1 s −1 at 20 °C) is observed for more reactive p ‐methoxystyrene.…”

Cationic polymerization of p-methylstyrene using various metal chlorides: design rationale of initiating systems for controlled polymerization of styrenes

“…In several past studies, researchers have successfully suppressed such side reactions through the elaborate design of initiating systems using Lewis acid catalysts (for example, BCl 3 , 1 SnCl 4 2-4 and TiCl 4 5-7 ) in conjunction with suitable additives, which demonstrates the controlled cationic polymerization of St. In addition, these initiating systems have been reported to be efficient for the controlled polymerization of substituted Sts, such as 2,4,6-trimethylstyrene, 8 p-methylstyrene (pMeSt) [9][10][11][12][13] and p-chlorostyrene (pClSt). [12][13][14][15] The design of initiating systems for controlled cationic polymerization is remedied by the careful choice of an appropriate catalyst and additive.…”

Can Propagation Reaction in Carbocationic Polymerization and Copolymerization of Styrene be Diffusion Controlled ?

“…[10] In the present paper, we are planning to show that for p-methylstyrene ( pMeSt), styrene (St) and p-chlorostyrene ( pClSt) diffusion controlled propagation reactions are incompatible with the reactivity ratios measured for the copolymerization between these monomers and with pmethoxystyrene ( pMeOSt). We will show also that a k ± p % 10 9 L mol À1 s À1 (and an equilibrium constant of ionization K i ¼ 9 10 À8 L mol À1 ) measured by the DC method for pMeSt [11] is incompatible with a k [12] in the same solvent at À30 8C.…”

“…It was extended later by R. Faust et al to St, [6] pMeSt [11] and pClSt [18] with k ± p between 10 9 and 4 10 9 L mol À1 s…”